1. Field of the Invention
The present invention relates to a zoom lens and an optical apparatus including the zoom lens, for example, to a zoom lens suitable for an optical apparatus such as a silver halide film camera, a digital still camera, a video camera, a telescope, binoculars, a projector, and a copying machine.
2. Description of the Related Art
A zoom lens to be used for an optical apparatus is required to have a short total lens length (length from a first lens surface on an object side to an image plane), a small optical system size as a whole, and high optical performance. Besides, the zoom lens is required to have a small change in optical performance even under an environmental change (temperature change, pressure change, humidity change, or the like). In general, when the entire system size is intended to be more reduced in the zoom lens, various aberrations, for example, chromatic aberrations such as an axial chromatic aberration and a lateral chromatic aberration frequently occur to greatly lower the optical performance. Moreover, a variation in aberration more frequently occurs in zooming. As a result, it becomes more difficult to obtain high optical performance over the entire zoom range.
An optical system using an optical element (lens) made of an optical material having a high anomalous partial dispersion characteristic, for example, an organic material, has been proposed to reduce the occurrence of the chromatic aberration among the various aberrations. In U.S. Pat. No. 7,292,398, the chromatic aberration is corrected in a well-balanced manner by providing an appropriate refractive power to an optical element made of an organic material having high anomalous dispersion and using an optical element made of another material. In this manner, the chromatic aberration of the entire system is reduced. Moreover, by using a solid material, the change in optical performance depending on a temperature is reduced as compared with the case where a liquid material is used.
In general, the organic material has a larger change in optical characteristics with respect to the environmental change such as the temperature change or the humidity change as compared with glass. In order to cope with the problem of the larger change in optical characteristics, there is known an optical system which corrects the chromatic aberration and reduces the variation in optical characteristics due to the environmental change by appropriately combining optical elements respectively made of multiple optical materials. In U.S. Pat. No. 7,864,453, the change in optical performance with respect to the temperature change is reduced by appropriately setting a focal length, an Abbe number of a material, and an anomalous partial dispersion characteristic of an optical element made of a resin having the anomalous partial dispersion characteristic, and a focal length and an Abbe number of a material of an optical element made of a resin without an anomalous partial dispersion characteristic. In U.S. Pat. No. 8,203,799, a variation in the chromatic aberration is reduced by appropriately using a refractive index and an Abbe number of an organic material for setting.